Aviation gasoline



United States Patent AVIATION GASOLINE Robert E. Barnum, Roselle, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Application February 20, 1956 Serialv No. 566,383

2 Claims. (Cl. 44-56) The present invention is concerned with an improved aviation gasoline. In accordance with the present invention the improved aviation gasoline is secured by using critical amounts of aromatic hydrocarbons in conjunction with diisopropyl ether. The preferred aviation gasolines of the present invention comprise petroleum hydrocarbon mixtures containing critical concentrations of aromatics, particularly aromatics containing between 7 and 9 carbon atoms in the molecule in conjunction with critical proportions of diisopropyl ether. This application is a continuation-in-part of Serial No. 263,021, filed December 21, 1951, and now abandoned.

It is well known in the art to prepare aviation fuels by various methods and processes and to use with these aviation motor fuels blending agents, such as branched-alkylethers, amines and the like. The aviation gasolines of necessity have a designed boiling range from about 100 F. to 350 F. These aviation gasolines also, in order to be entirely satisfactory, must meet certain specifications with respect to the minimum octane number secured with a lean mixture and with a rich mixture. In addition, the fuels must meet other requirements, as for example, a minimum B.t.u. value available per pound,

an acceptable, tolerance of water contact without exces-- sive loss by solution in the water, a low freezing point and a low or negligible content of non-volatile gum or residue upon evaporation.

The use of branched-ethers for decreasing the knocking tendency of gasoline motor fuel was first disclosed in US. Patent 2,046,243. Among the branched ethers disclosed for that use are methyl-tertiary-butyl ether, methyltertiary-amyl ether, di-tertiary-butyl ether and other homologs having a total of from 4 to 8 carbon atoms in. the. molecule and having at least one branched-alkyl group. Diisopropyl ether, or isopropyl ether as it is commonly called, is typical of the branched ethers. of the prior art, which taught its use for blending with a motor fuel in concentrations. of to 50%. or more. of methyl-tertiary butyl ether for blending in aviation gasoline to attain. particularly desirable antiknock levels was again taught in US. Patent 2,409,746. However, the calorific value of methyl-tertiary butyl ether is considerably less than that of diisopropyl ether. Hence its use in an aviation gasoline is much less. desirable.

The branched ethers have high antiknock. value in. aviae tion gasoline, in both lean and rich. fuel-air mixtures. However, the ethers have lower calorific value than the hydrocarbons of which aviation gasoline consists and, since the range of flight ofv aircraft depends upon the calorific value of the load of fuel the aircraft can carry, it has not been practical to use the ethers in substantial proportions in such fuel. It is a well-established principle in the art that such fuel shall have at least 18,700 B.t.u. per pound as its netcalorific value. An aviation gasoline, of grade 100/130 as defined in the US. Specification MEL-F4572, was. tested and found to have 19,000 B.t.u. per pound, net heating value. A blend of 5% diisopropyl ether in that fuel had 18,880 B.t.u. per pound, net heating The usev value. It is therefore to be understood that in accordance with the present invention diisopropyl ether is blended in concentrations between 3% and 7% by voltune in aviation gasoline. Adding less than 3% ether provides no measurable increase in octane rating of the gasoline, while 3% ether increases the octane rating of a O.N. aviation gasoline to- 100.2. This octane increment is close to the limit of accuracy of the determination. On the other hand, adding more than about 7% of ether to the aviation gasoline, not only results in a composition having a calorific value of less than the required 18,700 B.t.u., but also results in a lowered rate of Increase of octane number; the octane blending value of the ether drops off at above 7% in an aviation gasoline.

Among the types of hydrocarbons, for example, paraffins, isoparafiins, naphthenes and aromatics, which may be present in aviation gasoline, the aromatic hydrocarbons have the lowest heating value per pound. It has therefore been accepted practice in the manufacture of aviation gasoline to limit its volumetric content of aromatic components, all or" which have less than 18,000 net B.t.u. per pound, to a maximum total concentration of 25% and usually to a concentration not exceeding 20% in a wholly hydrocarbon composition. Since the motor octane number of most of the aromatic hydrocarbons in the boiling range of aviation gasoline is as high as, or higher than, the octane number of most of the branched ethers and of diisopropyl ether in particular; and since the heating. valueof the ethers is lower than the heating value of the aromatics, it has not hitherto been practical to use, as blending agents together in aviation gasoline, both the. branched ether and the aromatic hydrocarbon type of components. As will be pointed out below a distinction must be observed. between specification aviation gasoline suitable for use in multi-cylinder aircraft engines, and experimental aviation fuels tested for antiknock value in single cylinder supercharged engines. In accordance with the present invention, aromatic hydrocarbons of 7 to 9 carbon atoms per molecule, and preferably those of 8 or 9 carbon atoms, are blended in aviationgasoline in a maximum total concentration of 15% of aromatics along with the diisopropyl ethers. Benzene. cannot be used because its freezing point is too high.

As has previously been noted, some branched ethers have high. antiknock value in aviation gasoline. Their blends have good water tolerance in terms of negligibleloss by transfer of blending agent from the gasoline phaseto the water phase. However, the blends can. and do dissolve more water, at any given temperature, than does. aviation gasoline of wholly hydrocarbon composition. In the processes of manufacture and of storage. aviation gasoline may become saturated with water, with which it may come into contact. Such a condition may be where aviation gas is stored over Water as in aircraft carriers. A typical aviation gasoline saturated at about 70 F. temperature contains about 0.003% water by volume. Blends of as little as 5% of branched ether in such an aviation gasoline contain at saturation at least 0.006 and up to 0.01% of water by volume,

The solubility" of water in aviation gasoline and in blends, of. branched. ethers. with aviation gasoline de creases markedly with decreasing temperature. The solubility of water is decreased about ten-fold when the temperature is decreased from 100 F. to 0 F. A drop ofv temperaturev of, 1.00 degreesv or more is normally encountered in; the fuel systems of aircraft between ground, level, and, high. altitude inflightz. When the temperature,

drops, the water;.which was in solution at the higher temperature, isprecipitated at the lower temperature. When the lower temperatureis below 32" R, the water is precipitated as a solid, in the form of crystals of ice or of snow.

This phenomenon is particularly observed in the case of diisopropyl ether. The latter is a particularly desirable constituent of aviation gasoline as pointed out but hitherto this material, though in large supply from refinery operations, has not been able to be utilized for this purpose because of the icing problem. On the other hand, more costly ethers, such as ethyl tertiary butyl ether, do not exhibit this icing tendency. Therefore, the invention has for its main purpose the utilization of diisopropyl ether in aviation gasolines in the presence of water. Ethyl tertiary butyl ether in 5% concentration forms no ice when present in aviation gasoline saturated with water at no temperature as low as -76 F.

The fuel systems of aircraft are provided with fine filters of 80 mesh per inch, for example, or even with pores of only 10 microns diameter; which are quickly plugger by ice crystals, with disastrous results for the aircraft when the flow of fuel is thus suddenly interrupted. The greater the initial content of water in the fuel, the greater is the weight of ice precipitated from a given volume of fuel or within a given time of fuel fiow through the filter. From blends of branched ethers in aviation gasoline, not only is the amount of solidified water thus precipitated greater than for ordinary aviation gasoline, but also the water normally solidifies in the form of flaky snow with greater tendency for plugging filters than fine ice crystals would have.

Table I demonstrates that it is possible to add 5% of diisopropyl ether to commercial aviation gasoline of 100/130 grade and meet all requirements of the Defense Department specification for aviation gasoline MIL-F-5S72. It should be further noted that when dry ether is employed for this purpose a gasoline passing the freezing point requirements is obtained. As will be shown below this is not true when diisopropyl ether in 5% amounts is employed in conjunction with aviation gasoline that is saturated with water.

TABLE I Quality aspects of using diisopropyl ether 5% Diiso- Aviation Gasoline No propyl Specification MIL- Ether Ether F-5572 Tctraethyl lead, MlJGal 2. 96 2. 98 4.60 Max. Reid I1Yapor Pressure, Lbs./ 6. 2 5. 7 5.5-7.0.

q. ASTM Gum, Mgs./100 mi 1. 0. 8 3 0 Max 16 Hr. Accel. Gum:

Gum, Mg./100 ml 1. 8 1. 9 6.0 Max.

Ppt.. Mg./100 ml 0 0 2.0 Max. Copper Strip Corrosion, 3 Hrs. Pass Pass None. Sl. Disc 212 F. Copper Strip.

Freezing Point, F 90 90 76. Test f?! Water Tolerance, Mi. 0 2.0 Max.

os Net Heating Value, B.t.n./Lb 19, 013 18,877 18,700 Min. ASIM Distillation (at 4 to cc./Min.):

10% Evap. at T. 154 154 167 Max.

40% Even. at F 188 187 167 Min.

50% Evap at F. 201 198 221 Max.

90% Evap. at 241 239 275 Max.

End Point. 335 334 338 Max.

Sum of 10 355 352 307 Min.

Pts., F. Dist. Loss, Percent 1.2 0.5 1.5 Max. Resid. After Dist., Percent. 0.8 1.0 l eMaxr 100 Octane No. in lean mixture (12 to 14 lbs. air per lb. of fuel). 35,} 130 Performance No. in rich mixture (9 to 10 lbs. air per lb. of fuel).

Among the types of hydrocarbons which may be present in aviation gasoline in substantial proportions, the aromatics have the greatest tendency to dissolve water. At 100 F., for example, benzene and toluene dissolve about 0.45 mol percent water, whereas the naphthenes and isoparaflins in aviation gasoline dissolve about 0.13

mol percent water. At 50 F. the comparable solubilities of Water are, respectively, 0.17 mol percent and 0.035 mol percent. Since a high degree of solubility of water in aviation gasoline at ordinary temperatures is normally undesirable for the reasons previously stated and since both the branched ether and the aromatic hydrocarbon type of components tend to dissolve more water than does ordinary aviation gasoline, it has not hitherto 1 been practical, for these additional reasons to use both those types of components together in blends of aviation gasoline saturated with water.

It has now been found that if aromatics are used in conjunction with diisopropyl ethers in controlled amounts in water saturated aviation gasoline, all the specifications and all the quality requirements may be met and the tendency to ice is corrected.

The present invention may be more readily understood by the following examples.

EXAMPLE 1 The particular value of C and C aromatics for this effect upon the icing tendency of water-saturated gasoline is contrasted with the effect of C and C aromatics in the following table, showing the results for /2 hour cooling of gasoline to -50 F., of water-saturated gaso line at cc./min./sq. inch flow rate through 10 micron filter.

Efiect of aromatics on low temperature filterability of aviation gasoline Time to Ping Filter. Mins.

Vol. Percent Added Aromatic In Blend 0 {Xylenes 40 100+ 100+ 100+ 5 Ethyl Benzene 68 100+ C {Isopropyl Benzene. 45 100+ Trimethyl Ben ewes Cm Sec.-Butyl Ben one 40 Cu Triethyl n 18 ing below the required calorific content necessary to meet aviation gasoline specifications.

The preferred C and C aromatics comprise para,

meta and ortho xylene, ethyl benzene, n-propyl-benzene, isopropyl benzene, para, meta, and ortho ethyl-toluene, pseudo-cumene, mesitylene and hemi-millitene. The quantity of diisopropyl ether utilized must be from 3% to 7% based upon total gasoline.

EXAMPLE2 The aviation gasoline of the present invention usually also contains inhibitors, dyes, and from 1 to 4.6 cc. tetraethyl lead. T.E.L. is supplied in the form of a fluid containing alkyl halide scavenging agents. Other addi tives may be included if desired. A typical satisfactory composition in accordance with the present invention contains about 0.008% water and has the following components:

The aromatics content of the virgin naphtha is about and the total concentration of aromatics in the aviation gasoline is about 12.1%. It is to be noted that all the constituents, save the aromatics have lower Motor Octane Numbers than the diisopropyl ether.

The gasoline contained 4.6 cc. of tetraethyl lead in the form of l-T ethyl fluid per gallon. The inhibitor used Was 1.0 pound of 2,6,di-tertiary butyl-para-cresol per 5000 gallons. Red dye was added to the gasoline in a concentration of 0.28 gram per 100 gallons. The gasoline as constituted above had a grade rating of 108/145.

EXAMPLE 3 At Lean At Rich M lxture Mixture-- Blending Blending Octane N 0. Index No.

Grade 100/130 100 130 Diisopropyl Ether. 129 248 Butenes Alkylate. 107 150 Toluene 101 250 EXAMPLE 4 The low temperature filterability of water-saturated aviation gasoline containing 5% diisopropyl ether and varying concentrations of aromatics was compared With that of a gasoline containing no ether. The gasolines were saturated with Water at room temperature, chilled down to F. within /2 hour and filtered through a standard aircraft filter medium having pores of 10 micron diameter. The rate of fuel flow was comparable with the actual rate of flow in aircraft fuel systems. The rate was 100 cc. per minute for each square inch of filter 6 were carried out as a series of temperature levels, with the results shown below:

Low temperature filterability of diisopropyl ether blends [Filtration through 10 micron filter (0.4 sq. in. area) located 3'6 ofi tank bottom. Fuel mildly agitated during cooling and filtering] Percent Ditso- Percent Avia. Percent Alkylpropyl Ether Toluene ate+Virgln Base Blend A 5 o Blend B 5 I5 90 [Efieet of filtration temperature on plugging time cc. min./sq.in., l hour cooling rate] in. No Plug No. Plug (106 No Plug Mins.). Mins.). Mins.).

EXAMPLE 6 To show the adverse eifects upon the low temperature characteristics of water-saturated aviation fuel compositions when diisopropyl ether is employed with more than the critical amount of aromatics, the following data are presented. Blends of isooctane, diisopropyl ether and aromatics C and/or 0; were shaken vigorously with Water and stored over water for 24 hours. Thereafter the freezing point was determined. The latter is defined as the temperature at which the formation of a solid phase or crystals first become visible in a test tube. Since the specifications for aviation gasoline require a maximum freezing point of 76 F., it is customary to report only below 76 F. when a sample asses specification.

Diisopropyl ether, V01. Percent Isooctane, Aromatics, Crystals,

percent Percent F.

The above data also demonstrate clearly that when only slightly more than the critical amount of aromatics is employed together with the diisopropyl ether, the freezing point of the water-saturated blends rises precipitously. These are blends of pure hydrocarbons, not adjusted to proper volatility for use as commercial aviation gasoline.

EXAMPLE 7 Though methyl tertiary butyl ether, because of its lower calorific content, is not a desirable constituent for aviation gasoline, it behaves like diisopropyl ether in low temperature characteristics in water saturated fuel blends when associated with low and high concentrations of C surface.

Composition of gasoline, vol. percent:

Aromatics (mixed C and 0 O 5 15 25 Alkylate and Virgin base (2/1 ratio) 95 90 80 75 Diisopropyl Ether 5 5 5 0 Vol. percent water at room temp.

saturation 0.006 0.007 0.008 0.005 Mins. elapsed before filter plugs at -20F 20 No No No p s p s p s No plug indicates that all the gasoline sample was pumped through the filter during an elapsed time of 1 /2 to 2 hours without interruption by filter plugging at 20 F.

EXAMPLE 5 As a further demonstration of the beneficial eifects obtainable by the composition of the invention, low temperature filterability tests with diisopropyl ether blends aromatics. This is readily apparent in the data below:

Methyl tert. Butyl Ether, lsooetano, Aromatics, Crystals,

Vol. Percent Percent Percent F.

What is claimed is:

1. An improved aviation gasoline adapted for filterability at temperatures below the freezing point of water present therein and having a net calorific value of at least 18,700 B.t.u. per pound and having an initial boiling point of about 100 F. and a final boiling point of References Cited in the file of this patent about 350 F. comprising essentially petroleum hydro- UNITED STATES PATENTS carbon constituents containing between about 3% and 7% by volume of diisopropyl ether and 3% to 15% of aro- 2409156 Schulze et 1946 matic hydrocarbons having from 7 to 9 carbon atoms, 5 2409746 Evans et 0x22 1946 the ratio of said ether to said aromatic hydrocarbon be- OTHER REFERENCES ing greater than 5 19, said gasoline being further char- Aviation Gasoline Manufacture, by Van Winkle acterized 1n containing at least 0.006% by volume of first edition McGraWHm Inc. pages 41, 214, 216, 240

liquid water.

2. The composition of claim 1 containing from 0.00s 10 and 1944' to 0.01% water. 

1. AN IMPROVED AVIATION GASOLINE ADAPTED FOR FILTERABILITY AT TEMPERATUREE BELOW THE FREEZING POINT OF WATER PRESENT THEREIN AND HAVING A NET CALORIFIC VALUE OF AT LEAST 18,700 B.T.U. PER POUND AND HAVING AN INITIAL BOILING POINT OF ABOUT 100* F. AND A FINAL BOILING POINT OF ABOUT 350* F. COMPRISING ESSENTIALLY PETROLEUM HYDROCARBON CONSTITUENTS CONTAINING BETWEEN ABOUT 3% TO 7% BY VOLUME OF DIISOPROPYL ETHER AND 3% TO 15% OF AROMATIC HYDROCARBON HAVING FROM 7 TO 9 CARBON ATOMS, THE RATIO OF SAID ETHER TO SAID AROMATIC HYDROCARBON BEING GREATER THAN 5/19, SAID GASOLINE BEING FURTHER CHARACTERIZED IN CONTAINING AT LEAST 0.006% BY VOLUME OF LIQUID WATER. 